This invention relates to an ethylene-alpha-olefin rubber composition with improved damping characteristics which can also be molded, bonded to metal, and used as the vibration absorbing element in torsional vibration dampers, engine mounts, or other such vibration control devices. This invention also relates to a crankshaft damper for absorbing torsional and bending vibrations utilizing the improved rubber composition as the vibration absorbing element. U.S. Pat. No. 6,386,065, which is incorporated herein by reference, discloses an example of a torsional vibration damper to which the subject invention could be applied. Additional examples of crankshaft dampers and examples of rubber compositions for vibration damping to which this invention could be applied are disclosed in Pub. No. US 2002/0162627, which is incorporated herein by reference.
Rubber compositions are in wide use in vibration control devices. Diene elastomers such as NR, BR, SBR, IIR, CR and NBR have traditionally been used because of their low cost. They are generally vulcanized by means of heat-activated cure systems comprising sulfur and sulfur-based cure accelerators. Rubber formulated with these elastomers is generally very limited in terms of heat resistance and ozone resistance. As performance demands have increased in many applications, such as the increase in under-hood temperatures in automotive applications, higher performance elastomers such as EPM, EPDM, HNBR, AEM, fluoro- and silicone rubbers have increased in use. EPM and EPDM, members of the ethylene-alpha-olefin family of elastomers, are desirable for vibration dampers because of their high heat resistance, ease of incorporating fillers, and relatively low cost. EPDM and EPM are also desirable because they can readily be cured with peroxide cure systems which are known to provide better compression set properties, better heat resistance, and better compatibility with certain metal-adhesive coagents than sulfur cure systems. Unfortunately for some vibration damping applications, ethylene-alpha-olefin elastomers tend to be very resilient, low-damping elastomers.
For vibration damping, the most important property of the rubber composition is the degree of damping. One way to characterize the damping of rubber is to measure the ratio of loss modulus to storage modulus, known as “tan δ”, by dynamic mechanical testing. Typical tan δ values for EPDM compositions are in the 0.05 to 0.1 range. What is needed in some vibration control devices is a peroxide-cured ethylene-alpha-olefin composition with improved, higher damping and tan δ values, e.g., around 0.2 or higher. An increase in damping of around 100% or more is desired.
In the article “Low Modulus, High Damping, High Fatigue Life Elastomer Compounds for Vibration Isolation,” Rubber Chemistry & Technology, 57(4) 792–803 (1984), authors M. A. Lemieux and P. C. Killgoar, Jr. describe their attempts to increase the damping of sulfur-cured NR compositions and NR/BR blend compositions for automotive suspension applications. Known methods of increasing damping include adding more filler, decreasing the amount of curative, changing the amount of plasticizer and changing elastomers. Because of the limitations of each of these known approaches and the negative effects on the overall balance of properties of the rubber, other less predictable methods must often be tried. They focused on elastomer blends. They found that adding various grades of polyisobutylene (“PIB”) having a viscosity average molecular weight of about 900,000 or more to the NR and NR/BR compositions did not increase the damping at all, but adding BIIR did have the desired effect.
U.S. Pat. No. 5,310,777 discloses a rubber composition with very high damping. In that disclosure PIB is the primary elastomer component, to which are added rather large amounts of white filler and carbon black to obtain tan δ ranging from 0.4 to 1.5. The viscosity average molecular weight of the PIB in that disclosure should be above 300,000, preferably above 1,000,000, in order to accommodate the large amount of filler required to obtain high damping. Since PIB can not be crosslinked, that disclosure suggests adding a crosslinkable secondary elastomer and appropriate curatives to the composition. However, peroxide curatives are not suggested. Elastomer blending is highly unpredictable, and no prediction was offered as to what level of tan δ will be obtained when adding a crosslinkable elastomer to PIB.
U.S. Pat. No. 6,255,401 discloses a composition with increased hysteresis loss for tire tread, wherein PIB or isobutylene-isoprene copolymer (IIR) or isobutylene-styrene copolymer (SIR), having a weight average molecular weight in the range 2000 to 50,000, is added to NR or SBR or other elastomers to increase hysteresis loss. The weight average molecular weight is believed to be very close to the viscosity average molecular weight for these materials. Only sulfur-cured compositions are suggested, and only relatively small improvements in tan δ (increases of up to 18 per cent), are demonstrated.
Another similar example is disclosed in U.S. Pat. No. 6,060,552. The examples therein show that IIR of viscosity average molecular weight around 500,000 or PIB of viscosity average molecular weight around 2,000,000 added to SBR at levels of up to 1:1 by weight only increase tan δ by as much as 20 per cent. Again, only sulfur-cured examples are provided or suggested.
It is not surprising that the art does not suggest blending PIB or IIR with a peroxide-cured elastomer such as EPDM. It is well known that PIB and IIR are severely degraded by free-radicals.
After extensive evaluation of many ingredients and blends, the present invention unexpectedly meets the need for a free-radical-cured ethylene-alpha-olefin rubber composition with increased damping for use in vibration dampers, in other vibration control devices, and in engineered rubber products subject to dynamic loading such as belts and hose. The improvement desired for vibration dampers is to approximately double the normal value of tan δ.